In conventional ultrasonic microscopes, an acoustic lens is used. An ultrasonic wave is focused into a micro-spot and directed to a sample through the acoustic lens. A reflected wave from the sample is received and converted into a received electric signal. A part of this received signal is gated to pick up a portion of the sample-reflected wave. The peak value of the sample-reflected wave component thus picked up is detected to obtain a piece of information relating to a point on the sample. When this process is carried out while scanning the sample with ultrasonic waves and in the horizontal direction, the ultrasonic wave image of the sample can be obtained.
The peak value or strength of the detected value becomes the largest when the acoustic lens is focused on the sample. Some automatic focusing methods of the acoustic lens which use this fact have been proposed. In a case where the focal position of the acoustic lens is previously known, for example, the timing at which the reflected waves from the sample surface are received at the time the acoustic lens is focused can be calculated as a delay from the reference time such as the signal transmitting time. When the gating is carried out at this timing, therefore, the reflected wave from the sample can be picked up and its strength can be measured. The strength of the reflected wave from the sample is compared with a threshold value while moving the acoustic lens from a position remote enough from the sample to the sample. When these two values coincide with each other or the value measured exceeds the threshold value, the acoustic lens is stopped. The automatic focusing of the acoustic lens can be achieved in this manner.
The threshold value is made a little smaller than the strength of the reflected wave at the focus point. Using two gates, the threshold value is compared with peak values in the two gates. The automatic focusing of the acoustic lens can also be achieved in this manner. More specifically, the first gate is set at the time when the wave reflected from the sample surface at the focus point is received, and the second gate is set near this time. There are then used a circuit for checking that the strength of the detected signal of that reflected wave picked up by the first gate becomes larger than that picked up by the second gate, a circuit for checking that the strength of the detected signal at the first gate exceeds the threshold value, and an AND circuit for the outputs of the both circuits. The acoustic lens can be stopped responsive to output applied from the AND circuit.
In the case of the above-described focusing methods, however, it is necessary that the threshold value is set at a level near the strength of the reflected wave obtained at the time when the acoustic lens is focused on the sample. In addition, the threshold value must be adjusted to an appropriate level every time the sample is replaced by a new one because the strength of the reflected wave from the sample depends upon samples viewed.
The present invention is therefore intended to eliminate the above-mentioned drawback.
Accordingly, the object of the present invention is to provide an ultrasonic microscope, enabling the operator to more easily achieve automatic focusing but making it unnecessary for the operator to adjust the threshold value every time the sample is replaced by a new one.